From an early age children help others yet the underlying mechanisms of children's prosocial attention remain understudied. Comparing the attentional and physiological mechanisms of prosocial attention of typically developing and atypically developing children contributes to our understanding of the ontogeny of prosocial development. We presented typically developing (TD) children and children with Autism Spectrum Disorder (ASD), who often have difficulty developing prosocial behaviour, with scenarios in which an adult needed a dropped object to finish a task but was subsequently not helped by a second adult. In a perceptually matched non-social control scenario, children saw self-propelled objects move and drop without any adult present in the scene. Results showed a dissociation between arousal (pupil dilation) and the anticipation of the individual's need (gaze patterns), such that only TD children looked longer at the correct solution to the adult's need prior to the resolution of the situation. In contrast, following the resolution of the scene, both groups showed greater arousal when the adult was not helped compared to when the non-social situation remained unresolved. For the ASD group, this effect was greatest for children with higher developmental quotients. These results suggest that, despite similarities in prosocial attention between TD and ASD children, previously documented reduced prosocial behaviour in children with ASD may be in part due to a specific impairment in anticipating prosocial behaviour.